I think the demand would most certainly be there on a matchday, Penz. I don't think you can compare a proposed facility attached to the stand with the standard pub in the street. If Brent did it properly, and made it adaptable enough in terms of it being able to cater for events (and be a proper revenue stream) during the week, I don't see that as being too detrimental to any green space in the park.
But as I say, I would go big. Bolting on some sort of fanfest or Far Post extension would only make for a continued clique situation. The fanfest regulars are all now pretty much on a first name basis. Same faces, same seats, same pasties etc. By the same token, the Far Post crew could probably be identified by the shape of the indentation their arse makes in the seat, when they get up to empty their bag.
A bar something on the lines of Shearers at St James Park, would be a massive draw, and if done properly, could be a focal point for many hundreds of fans.